In gas turbine systems, fuel and air are combusted in a combustor of the system to generate high temperature, high pressure working gases. The turbine converts the expansion of the working gases over the turbine blades into mechanical energy, which then can be used to do useful work such as generating electricity.
It is generally known that increasing the temperature in the reaction zone of the combustor can enhance the efficiency of the gas turbine systems. It is also generally known that the formation of oxides of nitrogen (NOx) increases with the peak temperature in the combustor. Dry-low NOx (DLN) gas turbine systems minimize the undesirable NOx formation by premixing fuel and air before combustion so that the temperature stratification in the combustion zone is significantly reduced to reduce the peak temperature and the temperature field within the combustor is as uniform as possible.
One of the major constraints for advanced DLN combustor development is combustion dynamics, i.e. acoustics-related dynamic instability during combustion operation. High amplitudes of dynamics are often caused by the fluctuations in temperature fields (heat release) and pressure oscillations within the combustor chamber. Such high dynamics can impact hardware life and system operability of an engine, leading such problems as mechanical and thermal fatigue, which lead to hardware damage, system inefficiencies, unexpected flame blowout, and compromise in emission performances.
There have been multiple attempts to mitigate combustion dynamics, so as to prevent degradations of combustion performances. Conventionally, the basic methods in an industrial gas turbine combustion system include passive control and active control. Passive control refers to the usage of combustor hardware design features and characteristics to reduce either dynamic pressure oscillations or heat release levels or both. On the other hand, active control can be achieved through the introduction of pressure or temperature fluctuations, which are suitably controlled, to adjust the coupling between heat release and pressure oscillations so as to reduce amplitudes of combustion dynamics.
It is known that combustion dynamics are increased when the heat release and pressure fluctuations are in phase. Therefore, common solutions to mitigate dynamics are featured with dephasing the heat release and pressure fluctuations in the combustor. One representative apparatus used to address some dynamics concerns in gas turbine combustors is a resonator. However, its application has been limited to the attenuation of high frequency (i.e. greater than 1000 Hz) instabilities by pure absorption of acoustic energy. In addition, the installation of a resonator is accompanied with air management, which sometimes is not desirable for premixing designs for low emission performance.
Thus, it is desirable to provide a premixing apparatus that minimizes the combustion dynamics while retaining the low emission characteristics without introduction of pure dynamics-mitigation apparatus.